Inherent photoluminescence Stokes shift in GaAs

The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 μm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter

Photoluminescence lineshape of ZnO

The merger of the absorption coefficient dispersion, retrieved from transmission by the modified Urbach rule introduced by Ullrich and Bouchenaki [Jpn. J. Appl. Phys. 30, L1285, 1991], with the extended Roosbroeck-Shockley relation reveals that the optical absorption in ZnO distinctively determines the photoluminescence lineshape. Additionally, the ab initio principles employed enable the accurate determination of the carrier lifetime without further specific probing techniques

Orthogonal printing of uniform nanocomposite monolayer and oriented organic semiconductor crystals for high-performance nano-crystal floating gate memory

Inkjet printing is of great interest in the preparation of optoelectronic and microelectronic devices due to its low cost, low process temperature, versatile material compatibility, and ability to precisely manufacture multi-layer devices on demand. However, interlayer solvent erosion is a typical problem that limits the printing of organic semiconductor devices with multi-layer structures. In this study, we proposed a solution to address this erosion problem by designing polystyrene-block-poly(4-vinyl pyridine)-grafted Au nanoparticles (Au@PS-b-P4VP NPs). With a colloidal ink containing the Au@PS-b-P4VP NPs, we obtained a uniform monolayer of Au nano-crystal floating gates (NCFGs) embedded in the PS-b-P4VP tunneling dielectric (TD) layer using direct-ink-writing (DIW). Significantly, PS-b-P4VP has high erosion resistance against the semiconductor ink solvent, which enables multi-layer printing. An active layer of semiconductor crystals with high crystallinity and well-orientation was obtained by DIW. Moreover, we developed a strategy to improve the quality of the TD/semiconductor interface by introducing a polystyrene intermediate layer. We show that the NCFG memory devices exhibit a low threshold voltage (100 cycles), and long-term retention (>10 years). This study provides universal guidance for printing functional coatings and multi-layer devices

Microextensive Chaos of a Spatially Extended System

By analyzing chaotic states of the one-dimensional Kuramoto-Sivashinsky equation for system sizes L in the range 79 <= L <= 93, we show that the Lyapunov fractal dimension D scales microextensively, increasing linearly with L even for increments Delta{L} that are small compared to the average cell size of 9 and to various correlation lengths. This suggests that a spatially homogeneous chaotic system does not have to increase its size by some characteristic amount to increase its dynamical complexity, nor is the increase in dimension related to the increase in the number of linearly unstable modes.Comment: 5 pages including 4 figures. Submitted to PR

Phase turbulence in Rayleigh-Bénard convection

We present a three-dimensional simulation of Rayleigh-Bénard convection in a large aspect ratio Γ=60 with stress-free boundaries for a fluid Prandtl number σ=0.5. We find that a spatiotemporal chaotic state (phase turbulence) emerges immediately above onset, which we investigate as a function of the reduced control parameter ε. In particular we find that the correlation length for the vertical velocity field, the time averaged convective current, and the mean square vorticity have power law behaviors near onset, with exponents given by -1/2, 1, and 5/2 respectively. We also find that the time averaged vertical velocity and vertical vorticity fields have the same (disordered) spatial characteristics as the corresponding instantaneous patterns for these fields, and that there is no long-term phase correlation in the system. Finally, we present simple theoretical explanations for the time averaged convective current as a function of the control parameter, and for the fact that the time dependence of three global quantities (characterizing the dissipation of kinetic energy, the release of internal energy by buoyancy, and entropy flow) is essentially the same. ©2000 The American Physical Society.This work was supported by the National Science Foundation under Grant No. DMR-9813409. H.W.X. was also supported by the Research Corporation under Grant No. CC4250. One of us ~J.D.G.! wishes to acknowledge support from IBERDROLA during part of this project. X.J.L. also knowledges the support of the National Science Foundation under Grant No. DMR9876864 during the preparation of the manuscript. Numerical work reported here was carried out on the Cray-C90 at Pittsburgh Supercomputing Center and Cray-YMP8 at the Ohio Supercomputer Center.Peer Reviewe

Relation between Debye temperature and energy band gap of semiconductors

The work addresses an unresolved topic in solid-state physics, i.e., the dependence of the Debye temperature (TD) on the energy band gap (Eg) of semiconducting materials. The systematic calculation of TD by using the ratio of sound velocity and lattice constant from the literature resulted in the relation TD∝exp(Eg). The exponential relationship is confirmed by a theoretical model based on the microscopic analysis of the electrical conductivity in metals and semiconductors

Optical Bandgap Definition via a Modified Form of Urbach’s Rule

We are reporting an esoteric method to determine the optical bandgap of direct gap materials by employing Urbach’s rule. The latter, which describes the slope of the band tail absorption in semiconductors, in its original version, cannot be employed to pinpoint the optical bandgap. Herein, however, we show that a modified form of Urbach’s rule defines the optical bandgap, and therefore, enables the accurate determination of the optical bandgap energy, which turns out to be identical with the threshold energy for the band tail absorption. The model further produces an explicit expression for the absorption coefficient at the optical bandgap energy